1. Field of the Invention
The present invention relates to electrofishing and, more particularly, to apparatus and method for minimizing the likelihood of injury to fish while inducing electrotaxis and narcosis.
2. Description of the Prior Art
Electric fishing has been known since at least 1863 when a British patent directed to the subject was granted. However, it was not until after World War I that serious attention began to be given to electric fishing or electrofishing. As a result of various studies and investigations, certain conclusions reached by various researchers have been taken to be truisms. Some of these conclusions are based upon inadequate data, unsubstantiated assumptions or insufficient inspection and analysis of the fish caught and are therefore suspect.
There are many uses for electrofishing which are carried out to the extent the equipment available permits and without inducing an unacceptable mortality rate. Electrofishing permits the capture and removal of fish population from one locality to another. It permits surveys or population estimates to determine the type and number of fish present and their size range. Such estimates may also uncover natural fluctuations in population and assess the impact of channelization. Electrofishing permits guiding the movement of fish such as keeping predators away from freshly planted fry, keeping fish away from electric power plant water intakes, keeping migrating fish away from specific areas en route and trapping fish (such as the sea lampreys in the Great Lakes). Biological sampling may be accomplished through electrofishing to collect brood stock, determine species composition, tabulate size and age characteristics. Food habits can be determined by collecting fish at feeding areas. Tagging and marking studies are readily carried out by acquiring fish by electrofishing, provided that the stresses of handling and shocking do not injure or kill too large a percentage. The prospect of injury and death is of a particular concern when one is dealing with endangered species of fish.
Electrofishing permits a determination of seasonal migrations. Fish can be readily counted by recording changes in underwater field strength due to passage of fish therepast. Electrofishing can be used as an anaesthetic to assist in treating sick fish, such as parasite control, and to quiet fish for handling. Electrofishing can be used for collecting floating invertebrates and electrofishing can be used to quickly and efficiently electrocute fish without degrading the commercial value of the catch since the fish scales remain intact.
During electrofishing with pulsed DC electric current, a fish will have several reactions, depending upon the field strength or density in which it finds itself and upon the frequency, shape and width of the pulses. The first reaction is that of frightening the fish. A second reaction is electrotaxis, the involuntary exercise of swimming muscles to draw the fish toward the source of electric current. The third reaction is narcosis when the muscles go limp and the fish rolls on its side; this permits netting and acquisition of the fish. The fourth reaction is tetanus which is an involuntary contraction of the muscles without interleaved relaxation and can result in death. A fifth reaction can occur if the white muscles of the fish are stimulated to the point of an epileptic seizure, thereby causing morphological trauma.
Since the inception of electrofishing for scientific purposes, there have been reports of injuries to fish due to exposure to electric stimuli. The injuries include compression of the spinal column, torn supportive tissues around various organs and broken blood vessels (hematomas). In general, these injuries have been thought to be the result of high current densities which may be encountered by the fish near an electrode.
In normal electrofishing practice, direct current or pulsed direct current is used because aquatic animals will move, in general, to the anode electrode. In the case of fish, this movement, electrotaxis, involves a pseudo swimming reaction. As a fish approaches the anode electrode, it encounters an exponentially increasing field strength. At some critical value of field strength, depending upon many physical factors, such as the water conductivity, the fish may cease electrotaxis action, enter a state of narcosis and then tetanus, a few feet from the anode electrode or very near it. Often, the critical state occurs a few feet from the anode electrode or very near it. In either case, the fish almost always drifts near to or may actually touch the anode electrode. The field strength within this zone causing tetany is very high and a significant flow of electric current through the fish occurs. This electric current is generally believed to stimulate and then overwhelm the neuromuscular system of the fish. It is believed that the overwhelmed neuromuscular system causes the above referenced trauma.
This view overlooks or disregards facts relating to muscle cells and proven in laboratory experiments by biology researchers. A neuromuscular response requires a minimum threshold level of external electrical stimulus before response. Once the existence of a response is established, the following principal factors determine the type of response: frequency of pulse, duration (or width) of each pulse and the shape of each pulse.
It is known and accepted that a nerve cell responds best to an almost instantaneous rise from zero to a maximum value in less than 1 ms. When a cell responds or "fires", contraction is initiated, which contraction will proceed without regard to further external stimulus. If the external stimulus is brief, the cell will relax in approximately 1/300 of a second and be ready for the next pulse stimulus. In the event the initiating pulse has a long on time, the cell becomes stressed due to lack of relaxation and it may remain contracted. Tetanus is the constant unrelieved contraction of a muscle cell.
Preferably, the leading edge of the pulse is sharp enough to fully stimulate the nerve cell. Should the leading edge represent a sloping gradual rise to the firing point of the cell, traumatization and tetanus will result. The trailing edge of the pulse can be either square or exponential in decay time. Should the slope of the pulse be sinusoidal or a linear decay, the cell might not relax and such lack of relaxation will lead to trauma and tetanus.
In a paper entitled "Influence of Electrofishing Pulse Shape on Spinal Injuries in Adult Rainbow Trout" (1988), Sharber and Carothers evaluated the effect of three wave forms of pulsed DC on spinal injury to 300 to 560 mm rainbow trout captured by electrofishing. The overall injury incidence was 50% with a significantly higher incidence (67%) in fish stunned with a quarter sine wave than those captured with either exponential or square waves (44% each). Smaller fish or fish in less conductive water appear to have a lower incidence of injury, as reported in other studies. In a recent study by the Alaska Department of Fish and Game, the incidence of spinal injury to rainbow trout having a fork length of greater than 400 mm was 50%. As the result of such high incidence, further electrofishing for population studies of large rainbow trout has been suspended in Alaska until an acceptable solution is found. The pulse rate employed was 60 pulses/second.
Based upon criticism from authorities in the field of electrofishing, it was suggested that a pulse frequency of 500 pulses/second having a width of 0.2 ms. would prevent the injuries caused by a pulse rate of 60 pulses/second. The basis for this argument resided in a contention that the muscle cells would not respond to the repetitive pulsation with their limited 300 contraction cycles per second and, without response, would not be traumatized to the extent that injuries occur. Based upon a further study performed and reported in a paper entitled "Electrofishing Induced Spinal Injury in Rainbow Trout" (Sharber, Carothers, Sharber), various experiments were performed changing the pulse frequency through a range of 15 to 512 pulses/second and a correlation between injury rate and frequency was established. Based upon this study, less than 3% of the trout were injured at a frequency of 15 pulses/second with the incidence of injury rising to 24% at 30 pulses/second. From 60 to 512 pulses/second, the injury rate occurrence was 42% to 61%, respectively. Accordingly, the experimental results did not support an authoritarian viewpoint.
Further authorities in the field have postulated that the intensity of the field attendant a relatively small electrode results in overwhelming electric currents in the fish and thereby traumatize the muscles of the fish leading to broken backs. As discussed in further detail in the above identified paper entitled "Influence Of Electrofishing Pulse Shape On Spinal Injuries In Adult Rainbow Trout", experimental results indicate that electrode size and shape, even though producing substantially different current gradients, had little or no influence on the rate of spinal injury.